I. Field of the Invention
The present invention relates to a retainer assembly for clamping a printed circuit board between spaced surfaces within a chassis, and particularly to a retainer assembly which facilitates entry and removal of a card from between those surfaces and which efficiently applies and relieves the clamping force between the board and the retainer.
II. Background of the Invention
Circuit board retainers have been developed to handle a broad range of applications. For example, such known retainers comprise a plastic guide that aligns the printed circuit board with an interface connector, or high-performance retainers that will captivate a printed circuit board under the most extreme shock and vibration conditions as encountered by military aircraft and spacecraft.
Of the known printed circuit board retainers one type, described in U.S. Pat. No. 4,823,951 comprises a partially threaded screw or rod and a plurality of members slidably mounted end-to-end on the rod. The members disposed on the rod have end portions which are wedge-shaped and engageable with one another and designed to move at least one of the members in a transverse direction relative to the rod when the members are moved towards one another along the rod.
The transverse direction in which at least one of the members is moved when the retainer is being locked acts to engage a clamping surface of that member against an edge of the printed circuit board. The opposite edge of the board is thereby brought into contact with and clamps against a spaced surface fixedly connected to or integral with the chassis in which the board is to be mounted. Typically, the rod in these prior art retainers is threaded on one end and the rod is rotated with an appropriate tool at the opposite end such that the threads of the rod engage with mating threads on a nut attached to one of the end-most members mounted on the rod. In this manner, the members are moved toward one another as the threaded rod is tightened.
In the above described retainers a problem is encountered upon unlocking of the retainer because the surface tension and friction between the clamping surface of the transversely movable member and the edge of the circuit board sometimes does not permit release of the board from the clamping surface of the member when the retainer is unlocked. Further contributing to this problem is the friction between the wedge-shaped end portions of adjacent members of the retainer. Since these wedge-shaped end portions engage with end portions of adjacent members to move at least one of the members into clamping engagement with the printed circuit board, when the retainer is unlocked the friction between the respective end portions sometimes restrains the members from returning to their unlocked positions thereby interfering with removal of the board from the chassis.
The amount of clamping force between the members of the retainer and the edge of the circuit board is dependent upon the torque applied to rotate the rod. Thus, for purposes of locking and unlocking the retainer to clamp or remove the printed circuit board from its chassis, efficient transfer and release of the torque, and hence the force applied to move the slidably mounted members, will result in improved operability of the retainer.
Printed circuit board chassis in most of the high-performance applications are sealed boxes which do not allow any air to pass over the circuit boards for the purpose of cooling the electronic components. The heat generated by these components is conducted through a metal heat sink attached to the printed circuit board and then transmitted to a heat exchanger or plenum. The critical thermal interface of the heat transfer path becomes the printed circuit board retainer. The amount of heat that can be transferred depends on the surface area of the retainer, the surface finish of the retainer, and the clamping pressure between the retainer and the printed circuit board.
It is, accordingly, an object of the present invention to provide a retainer and an assembly incorporating the retainer which efficiently transmits and releases forces applied between the rod and the slidably mounted members, and which acts to relieve surface tension between the board and the clamping surface of the members or the retainer.
Additional objects and advantages of the invention will be set forth in the description which follows and in part will be obvious from the description or may be learned by practice of the invention.